It is desirable to design the front bumper portion of a car so that when it is struck it "gives way" and then returns without damage. These types of bumpers are commonly known as shock absorbing bumpers. Currently in the United States, there exists Federally established minimum speed standards for shock absorbing bumpers. However, during a high speed impact, shock absorbing bumpers do little to dissipate the massive amounts of energy present. Thus, bumpers designed to absorb compressive forces during low speed impacts are ineffective to absorb the energy present during a high speed impact. Furthermore, with late model designs employing small engine compartments, crush space is decreasing making the energy absorbing capacity of the shock absorbing bumper even more critical.
There have been attempts to design a shock absorbing bumper that has an adjustable energy absorbing feature. For example, U.S. Pat. Nos. 5,011,205; 3,992,047; 3,848,914; 3,789,948; and 3,355,208 all employ the use of fluid filled cylinders for controlling the compression resistance of a vehicle bumper cylinder. While these devices are undoubtedly operative in accomplishing their objectives, they are expensive to produce, complicated in operation, and not compact.
Therefore, it is the object of the present invention to provide a cylinder having a variable compression resistance which is inexpensive to manufacture, simple in operation, and compact.